1. Field of the Invention
The present invention relates to a system and method for automatically controlling a vehicle speed to a desired cruise speed with a traction control function to prevent occurrence of slip on drive wheels of vehicles in which automatic cruise speed controlling systems are mounted.
2. Background of the Art
Various types of automatic cruise speed controlling systems have been proposed.
One of such automatic cruise speed controlling systems is exemplified by a Japanese Patent Application First Publication No. sho 60-4428 published on Jan. 10, 1985.
Other previously proposed automatic cruise speed controlling system are exemplified by two Japanese patent Application First Publications (Unexamined) No. sho 62-261545 published on Nov. 13, 1987 and No. sho 62-286845 published on Dec. 12, 1987 (the latter Japanese Patent Application First Publication No. sho 62-286845 corresponding to a U.S. patent application Ser. No. 057,086 filed on June 3, 1987), now U.S. Pat. No. 4,829,438.
In the latter two Japanese Patent Application First Publications, a microcomputer receives a vehicle speed indicative signal from a vehicle speed sensor when a set/cruise switch is depressed to turn on so that the microcomputer stores the current vehicle speed V as the set cruise speed and starts the automatic cruise speed control. The automatic cruise control is such that an opening angle of an engine throttle valve is adjusted through a throttle actuator so that the current vehicle speed coincides with the set cruise speed without operation of an accelerator pedal.
In addition, a table memory storing an engine torque (Te) data and another table memory storing a target opening angle (.theta..sub.s) of the throttle valve are provided. The microcomputer reads a current opening angle (.theta.) of the throttle valve from an opening angle sensor for detecting the current opening angle of the throttle valve and an engine speed (Ne) from a crank angle sensor and reads the engine torque (Te) from the table memory on the basis of the read opening angle (.theta.) of the throttle valve and read engine speed (Ne). Then, the microcomputer reads the target opening angle (.theta..sub.s) from an estimated running resistance (D) based on the read engine torque (Te) and the set cruise speed (V.sub.s). Then, the microcomputer issues a throttle valve actuate instruction to the throttle actuator so that the opening angle of the throttle valve (.theta.) coincides with the target opening angle (.theta..sub.s).
Thus, since the instantaneous running resistance is estimated with reference to the data on the engine performance and vehicle operation and an operating variable for the actuator is determined on the basis of an estimated value of the running resistance, a stable vehicle comfortability can be assured even if conditions of road on which the vehicle runs change.
However, since in such a previously proposed automatic cruise speed controlling system a speed of drive wheels (rear tire wheels in a case of front-wheel rear-drive vehicles) is adopted as the vehicle speed, the speed of the drive wheels does not coincide with the true vehicle speed (vehicle body speed) when the vehicle runs on a low frictional resistance road such as a road covered with snow or on a frozen road.
Suppose that the vehicle enters during the automatic cruise control operation on a road having a gradient of 3 percent and having a low friction coefficient (for example, .mu. = 0.1) (hereinafter, such a road is referred to as a low frictional road).
At this time, a slip occurs on the drive wheels during an interval of time for which the vehicle runs on the frictional road (time is between 10 seconds and 30 seconds) and a vehicle body speed (V.sub.CAR) is reduced. However, the drive wheel speed (V.sub.w) is maintained at the cruise speed so that a slip rate is increased. Consequently, a steering force and a cornering force during a steering wheel operation are lost.
Hence, it is necessary to give a priority higher than the automatic cruise speed control to prevention of occurrence of slips on the drive wheels.